Low mass spindle and z-axis unit

ABSTRACT

A spindle and Z-axis unit comprises a spindle housing and a hollow rotor shaft rotatable and axially movable with respect to the spindle housing. The rotor shaft carries a collet at its lower end for chucking a machine tool such as a drill bit. The tool is changed in the collet by pneumatic actuation of a diaphragm which actuates a push rod within the rotor shaft to move the collet axially against a spring bias to clamp and unclamp the tool. The shaft is moved in the Z-direction along with a thrust bearing assembly by a screw nut which actuates a road to move the rotor shaft in the Z-direction to drill a workpiece after a pressure foot has first been moved to clamp the workpiece with respect to the spindle. Thus, the moving weight or mass previously attributable to parts such as a spindle saddle and spindle housing can be eliminated without impairing the reliability and chucking force of the collet mechanism.

This is a division of application Ser. No. 480,940, filed Feb. 16, 1990.

BACKGROUND OF THE INVENTION

This invention relates to improvements in a spindle and Z-axis unit fordrilling machines and, more particularly, to a spindle unit whose movingweight or mass is substantially reduced to minimize driving power andnoise while, at the same time, permitting the drilling of largerdiameter, high quality holes and increasing acceleration control.

FIG. 1 shows a conventional spindle unit designated generally by thenumeral 14 widely used for drilling machines which drill holes inprinted circuit boards (PCBs). A hollow rotor shaft 101 includes abuilt-in copper core 110, a thrust flange 103, and a tapered collet 24for chucking a drill bit 25. The rotor shaft 101 is supported by aradial air bearing 102 and by an axial thrust air bearing 105 locatedinside of spindle body or housing 104. A selectively energized motorcoil 106 surrounding the core 110 drives the rotor shaft 101 for adrilling operation. An arrow 111 shows a direction of a supply of air tothe air bearings 102, 105 through an inlet port on the side of adiaphragm body at the top of the spindle housing 104. The direction ofan air supply for collet actuating diaphragm 107 mounted in a cavity inthe diaphragm body 108 is designated by arrow 112. In an automatic toolchange (ATC) process, the diaphragm 107 when actuated by the air supplyin direction 112 pushes the collet 24 downwardly axially relative to therotor shaft 101 in the direction of arrow 113 via a push rod 114 locatedconcentrically within the rotor shaft 101. This downward movement opensthe jaws of the collet 24 to allow an ATC operation for the drill bit25.

FIG. 6 and FIG. 7 show a conventional spindle of the type shown in FIG.1 mounted on a conventional Z-axis unit. A drive motor 7 is mounted on aunit base 6 to drive a screw shaft 11 supported by ball bearings 9 alsolocated on the unit base 6. Screw shaft 11 drives a screw nut 12 towhich a spindle saddle 13 is mounted for movement in an axial direction(the Z-axis) of the spindle 14. The spindle saddle 13 can be supportedand guided by a known linear guide mechanism The spindle saddle 13 hasthe conventional spindle unit 14 fitted therein and reciprocates thespindle unit 14 in the Z-axis direction, and also reciprocates anaxially movable pressure foot 20 with a chip evacuation system 39supported, axially guided and moved by brackets 15 mounted on thespindle saddle 13, a pair of air cylinders 16, swivel joints 17, andshafts 19 supported by bearings 18 located at both sides of the spindleunit 14.

In the typical drilling process, the spindle saddle 13 is caused to movedownwardly by actuation of the screw nut 12, and the pressure foot 20first contacts a surface of a PCB 22 to be drilled. The pressure foot 20clamps the PCB 22 by an actuation force of the air cylinders 16. Thespindle unit 14 is then advanced in the Z-direction through an openingin the pressure foot 20 and drills the PCB 22 via the drill bit 25 whichis rotated when the motor coil 106 is energized to rotate the rotorshaft 101. As soon as the tip of the drill bit 25 reaches a specifieddepth (the down limit), the pressure foot 20 and the spindle saddle 13retract to a resting position (the up limit), and a tooling table 21upon which the PCB 22 is secured and a spindle carriage (not shown) moveto the next drilling position to repeat the pressure foot clamping anddrilling process.

In the above described Z-axis unit, the total weight of the variousmoving parts exceeds 15 kg. Most of the weight is attributable to thespindle saddle 13 and the spindle body 104. Accordingly, highacceleration control which is necessary for high speed positioningcannot be attained satisfactorily and undesired noise is produced byexcessive G forces. Reductions in the size of the drive motor and otherspindle mechanisms have not been achievable up to now as a result.

FIG. 8 shows another conventional form of spindle unit designatedgenerally by the numeral 14' which uses a centrifugal collet 115 at theend of a rotor shaft 101' instead of a diaphragm and tapered collet ofthe type shown in FIG. 1 to reduce the moving weight of the unit forhigher acceleration control and noise reduction. Parts similar to thoseshown in FIG. 1 are designated with the same numerals but are primed.The rotor shaft 101' includes a built-in copper core 110' which isslightly longer than the core 110 shown in FIG. 1 for permitting axialmovement. Radial air bearings 102' are provided along the axial lengthof the rotor shaft 101', and a thrust flange 103' is integral with therotor shaft 101'. A thrust air bearing 105' in a thrust bearing assembly38 at the top of the unit 14' supports the flange 103'. A selectivelyenergized motor coil 106' surrounds the core 110'. A supply ofpressurized air is provided to the radial bearings 102' in the directionshown by arrow 111' through a port (unnumbered) in the spindle housingor body 104'. Unlike the spindle unit 14 in FIG. 1, however, the thrustair bearing assembly 38 in the spindle unit 14' of FIG. 8 is supportedand guided in the axial or Z-axis direction by a special radial airbearing 117' and an air groove 118' located on the radial periphery ofthe thrust bearing assembly 38 instead of through a relatively massivespindle saddle and the spindle housing. An arrow 116' shows thedirection of a supply of air for the thrust air bearing 105' through aport in the assembly 38. A rod 32 fixedly mounted on the thrust bearingassembly 38 moves the drill bit 25 in the axial or Z-directiondesignated by double-headed arrow 119 via the thrust air bearingassembly 38, the rotor shaft 101' and the centrifugal collet 115'. Withsuch an arrangement, the moving weight of the spindle unit 14' isreduced to about 1.5 kg. However, the centrifugal collet 115' in thisform of spindle has a chucking or clamping force which is unacceptablysmall at low speed ranges from about 15 to 30 Krmp. It is necessary inorder to achieve high hole quality (i.e. no roughness, no smearing) fordrilling hole sizes of 0.08" to 0.25" to maintain an adequate chuckingforce at the low speed range. As a result, although the total movingweight of the spindle unit 14' can be decreased with a centrifugalcollet 115' of the aforementioned type so as to increase accelerationcontrol and reduce noise, drill bit sizes are limited to 0.08" or lowerto ensure adequate chucking force.

FIG. 13 shows details of the centrifugal collet mechanism used in thespindle shown in FIG. 8. In particular, a front end of a rotor 101' issupported in radial air bearing 102' so as to rotate around the sameaxis as the spindle body 104'. A depression 3a is formed in the shaft101' and is open toward the front of the rotor with a suitable depth inthe axial direction. A guide portion 6a provided with a central fittinghole 5a to which a tool 25 fits is inserted into the interior of thedepression 3a. At the bottom of the depression 3a, a cylindrical guideportion 8a having a central fitting hole in which the tool 25 fits isinserted so as to extend axially. The guide portion 6a with the centralfitting hole 5a, the cylindrical guide portion 8a and the central bottomhole 7a formed on the bottom of the rotor 101' are adapted to beconcentrically fitted with an allowable clearance of several microns.

A centrifugal piece 13a provided with a fitting hole 12a to which theguide portion 8a is applied or fitted and another fitting hole 9athrough which the tool 25 is inserted is situated in the hole 3a withpredetermined clearances or gaps 10a and 11a to the inner surface of thedepression 3a and an outer surface of the tool 25. The centrifugal piece13a is statically held within the depression 3a so as to keep thepredetermined clearance 10a and 11a by means of an O-ring 14a situatedas shown around the guide portion 8a. The fitting hole 9a of thecentrifugal piece 13a through which the tool 25 is inserted is designedto have substantially the same fitting allowance as that of the centralfitting hole 5a and the central bottom hole 7a. The tool 25 is preventedfrom dropping out of these holes 5a, 7a and 9a when it is static orstable by means of another O-ring 15a fitted around the cylindrical bodyof the tool 25.

When the rotor 101' rotates during a drilling operation of the spindle,the centrifugal piece 13a eccentrically rotates together with the rotarymotion of the rotor 101' resulting in transformation of the rotarycenter of gravity due to eccentric rotation. As a result, a centrifugalforce is generated in the cylindrical guide portion 8a and the tool 25,and a torque or a force couple is generated, so that the tool 25 isfirmly held in place by a reaction force of the fitting faces of thefitting holes 5a and 7a, and friction coefficients of the fitting faces.Consequently, when the rotor shaft rotates at a speed less than 30 Krpm,it is difficult to obtain sufficient centrifugal force of the toolbecause the collet slips on the tool holding surface.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a spindle and Z-axisunit which overcomes the problems and disadvantages encountered inconventional units.

More specifically, it is an object of the present invention to provide aspindle constructed in such a manner whereby the weight or mass of themoving spindle parts is reduced and tee reliability of the colletmechanism at low speed ranges is increased.

Yet another object of the present invention is to improve dimensionaldrilling accuracy and thus productivity while, at the same time,permitting the range of drill hole sizes to be increased so as to extendfrom 0.004" to 0.25" in diameter and to provide an all-round drill bitrange.

A still further object achieved with the present invention is thereduction of power needed for the spindle drive motor which has theconsequent advantage of minimizing maintenance, improving reliability ofthe mechanisms and increasing cost performance.

The foregoing objects have been achieved in a spindle unit constructedin accordance with the present invention such that a stationary spindlehousing is provided which does not require the use of a relativelymassive, movable spindle saddle and which avoids a centrifugal collethaving low chucking forces in a low speed range. More particularly, ahollow rotor shaft with a built-in copper core is arranged within thespindle housing and is provided with a thrust flange which is supportedon a thrust bearing assembly via an axial air thrust bearing. The shaftis supported with respect to the housing via spaced radial air bearingsand is movable axially relative to the shaft along with the thrustbearing assembly. A motor coil surrounds the built-in core on the rotorshaft and is selectively energized to rotate the shaft during a holedrilling operation. A pneumatically actuated diaphragm arranged withinthe thrust bearing assembly is actuated to move a push rod axiallywithin the rotor shaft to engage the taper collet and disengage the toolduring a tool changing operation, after which the push rod is disengagedand the collet is spring biased back to its tool engaging positionwithin the shaft to chuck the tool with sufficient force.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects and advantages of the presentinvention will become more readily understood from the followingdetailed description of presently preferred embodiments when taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional elevation view of the above discussedconventional diaphragm-type spindle unit;

FIG. 2 is a top plan view of the unit shown in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III--III in FIG.1;

FIG. 4, is a cross-sectional view taken along the line IV--IV in FIG. 1;

FIG. 5 is a bottom plan view of the unit shown in FIG. 1;

FIG. 6 is a partial cross-sectional elevational view of a conventionalZ-axis unit for the spindle of FIG. 1;

FIG. 7 is a partial cross-sectional side view of the conventional Z-axisunit of FIG. 6;

FIG. 8 is a cross-sectional elevational view of a conventionalcentrifugal collet spindle unit;

FIG. 9 is a top plan view of the unit shown in FIG. 8;

FIG. 10 is a cross-sectional view taken along the line X--X in FIG. 8;

FIG. 11 is a cross sectional view taken along the line XI--XI in FIG. 8;

FIG. 12 is a bottom plan view of the unit shown in FIG. 8;

FIG. 13 is a fragmentary longitudinal cross-sectional view taken alongthe line XIII--XIII in FIG. 8;

FIG. 14 is a cross-sectional view of one embodiment of a spindle unitconstructed in accordance with the present invention;

FIG. 15 is a top plan view of the unit shown in FIG. 14;

FIG. 16 is a cross-sectional view taken along the line XVI--XVI in FIG.14;

FIG. 17 is a cross-sectional view taken along the line XVII--XVII inFIG. 14;

FIG. 18 is a bottom plan view of the unit shown in FIG. 14;

FIG. 19 is a fragmentary longitudinal cross-sectional view taken alongthe line XIV--XIV in FIG. 14;

FIG. 20 is a graphical illustration of supercritical run-outcharacteristics of high speed spindles;

FIG. 21 is a graphical illustration of sub-critical run-outcharacteristics of high speed spindles;

FIG. 22 is a cross-sectional elevations view of another embodiment of aspindle unit constructed in accordance with the present invention;

FIG. 23 is a top plan view of the unit shown in FIG. 22;

FIG. 24 is a cross-sectional view taken along the line XXIV--XXIV inFIG. 22;

FIG. 25 is a cross-sectional view taken along the line XXV--XXV in FIG.22;

FIG. 26 is a bottom plan view of the unit shown in FIG. 22;

FIG. 27 is a partial cross-sectional elevational view of an improvedZ-axis unit for a spindle of the present invention shown in FIGS. 14 and22; and

FIG. 28 is a side view of the Z-axis unit of FIG. 27.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings and, in particular, to FIG. 14, oneembodiment of a spindle in accordance with the present invention isshown and designated generally by the numeral 14". Parts similar tothose shown in FIGS. 1 and 8 are designated by the same numerals butwith a double prime. In particular, this spindle unit 14" has a rotorshaft 101" with an outer diameter d₂ and a built-in copper core 110",and a thrust flange 103" with a diameter d₁ integral with and at the topof the shaft 101". Radial air bearings 102" are spaced axially along therotor shaft 101". A thrust air bearing 105" arranged in the thrustbearing assembly 38" supports the flange 103". A selectively energizedmotor coil 106" surrounds the core 110". A supply of pressurized air tothe radial air bearings 102" is provided through ports in the spindlehousing 104" in the direction shown by the arrow 111". The pressurizedair is also supplied to the thrust bearing 105" through a port in theassembly 38" as shown by the arrow 116". Air is supplied to the bearings102", 105" at pressure P_(b) and also to air bearing 117" on the outerperipheral face of the assembly 38" through radial air grooves 118"arranged in the assembly 38" whose diameter is D₂. The spindle housingat the upper end has a diameter D₃. A rod 32" mounted on the assembly38" is actuated in the axial direction by a Z-axis unit (FIGS. 27 and28) and moves the drill bit 25 in an axial or Z-axis direction shown bydouble-headed arrow 119" by the above described air bearing system.

A spring-biased diaphragm 120" similar to but of smaller diameter D₁than diaphragm 107 in FIG. 1 is provided in a cavity of the thrustbearing assembly 38" and is supplied with pressurized air at pressureP_(d) through a port in the direction indicated by the arrow 112".During an ATC process, collet-actuating air at the pressure P_(d) issupplied through the port in the assembly 38" to the a diaphragm 120"which is connected with a push rod 114" to push a stopper 122" normallybiased upward by a disk spring 121 in the downward direction indicatedby arrow 113" and thereby open the taper collet 24" which is normallyspring biased toward a chucking position within the shaft 101" to allowchanging of the drill bit 25". The air supply pressure P_(b) for eachbearing 102", 105" is approximately 4.8 to 5.0 kg/cm², and the airsupply pressure P_(d) for the collet actuation diaphragm 120" isapproximately 5.5 to 6.0 kg/cm².

FIG. 19 shows details of the tapered collet mechanism used in thespindle depicted in FIG. 14. The main body of the rotor shaft 101' has acylindrical shape and a clamp or grip portion 28a provided with atapered hole 27a opening forwardly. The grip portion 28a is placed atthe front end of the cylindrical rotor 101" so as to extend outwardly.The tapered hole 27a is concentric with the rotor 101" and communicatedwith the interior of the rotor 101". The collet 24" is inserted into thetapered hole 27a formed in the clamp portion 28a in a manner to allowremoval from the inserted position. The collet 29a has a tool holdingportion 26a placed in the tapered hole 27a at the front end of the chuckso as to hold the tool 25 and an expanded portion or stopper 122" at itsrear end adapted so as to movably fit in the rotor 101". The toolholding portion 26a has a through-hole 29a opening to the front end ofthe holding portion 26a, through which the tool 25 is inserted. The toolholding portion 26a has a cylindrical shape and an axial slit 125"formed on the outer circumference of the portion 26a. When the collet24" moves toward the rear end of the spindle mechanism, the tapered hole27a of the grip portion 28a gradually becomes smaller, and consequentlythe tool holding portion 26a is clamped or pressed inwardly to close thethrough hole 29a. When the collet 24" is moved toward the front end, thetool holding portion 26a is released from its clamped condition due tothe tapered hole 27a opening outwardly. The disk spring 121 is adaptedto be placed in the space between the expanded portion or stopper 122"formed at the rear end of the collet 24" and a stepped front end portionof the rotor 101" in order to urge or press the collet 24" rearwardly.The resilient force of the disk spring 121 normally presses or urges thecollet 24" along the rearward axial direction, and the tool holdingportion 26a is kept at its closed condition. When the collet 24" movesforwardly by means of a collet opening-and-closing mechanism against theresilient force of the spring 121, the tool holding portion 26a opens.

Accordingly, the moving weight or mass with the arrangement of FIG. 14has been decreased from more than 15 kg to 2.8 kg. As a result,acceleration controls and noise reduction are greatly improved withoutany compromise in the drill bit sizes which can be used to obtain highquality holes. During the drilling process with the spindle unit 14" ofFIG. 14, only the rotor shaft 101", the collet 24", the drill bit 25,the push rod 114", the diaphragm 120" and the assembly 38" are driveninstead of the entire spindle unit as is the case with the unit 14 ofFIG. 1 which also requires movement of a spindle saddle 13. As a result,the spindle moving weight or mass is decreased to 2.8 kg, and spindlesaddle 13 of FIG. 1 can be eliminated.

FIG. 22 shows another embodiment of the spindle unit in accordance withthe present invention which is substantially the same as the embodimentof FIG. 14 but which has a smaller upper portion to further reduce themoving mass. Parts similar in construction and operation to parts inFIG. 14 are designated by the same numerals but are triple primed andwill not be further described except to the extent set forthhereinafter. The air supplied to the diaphragm 120'" is at pressure P₁,and the air supplied to the radial bearings 102'" and axial thrustbearings 105'" is at pressure P₂. The outer diameter of the rotor shaft101'" is d₂ ' and of the thrust flange 103'" is d₁ '. The diaphragm120'" has a diameter D₁ ', the assembly 38'" has a diameter D", and theupper portion of the spindle housing 104'" has a diameter D₃ '.

Therefore, if (d₁ ² -d₂ ²) P_(b) =(d'₁ ² -d'₂ ²) Pz [where P₂ is 5˜10kg/cm² ], and D₁ ² P_(d) =D'₁ ² P₁ [P₁ is 9˜10 kg/cm² ], the flangediameter d₁ ' is reduced to 75% of d₁, and the diaphragm diameter D₁ 'is reduced to 70% of D₁. Accordingly, D₂ ' and D₃ ' are also reduced tonear 70% of D₂ and D₃, and the moving weight is significantly reduced inthe FIG. 7 embodiment from 2.8 kg to 2.0 kg. As a result, the overalldimensions of the spindle unit 14'" are substantially reduced orminiaturized.

Ordinary typical high speed spindles have two types of dynamic run-outcharacteristic as shown in FIGS. 20 and 21. The dynamic run-outcharacteristic curves of FIG. 20 are known as the supercriticalcharacteristic, i.e. the critical run-out point between minimum andmaximum speed FIG. 21 shows curves of another run-out characteristicknown as the sub-critical characteristic, i.e. the critical run-outpoint over maximum speed. The characteristic curves shown by the solidlines A in FIGS. 20 and 21 are shifted rightwardly as shown by thedashed lines B as the pressure of the air supplied to the air bearingsis increased.

For example, with regard to FIG. 20, the critical point for thesupercritical characteristic shifts rightwardly from curve A to curve B.Thus, the supercritical run-out characteristic (measured in μm) issignificantly improved in the speed range of 60 to 80 Krpm (thousandrevolutions per minute) for small holes of from 4 to 40 mils and is onlyslightly worse in speeds between 80 to 120 Krpm. Futhermore, FIG. 21shows that the sub-critical characteristic is also greatly improved inspeed ranges at or above 80 Krpm so that the run-out reaches maximumonly at much higher speeds than is the case with conventional spindles.At the same time, however, adequate collet actuation chucking force canbe maintained for larger diameter drill bits.

FIGS. 27 and 28 show a presently preferred embodiment of the morecompact Z-axis unit used for the spindles shown in FIGS. 14 and 22. Morespecifically, drive motor 7' is mounted on the unit base 6' and drivesthe screw shaft 11' supported by ball bearing 10' and the housing 9'which is also mounted on the unit base 6'. The screw shaft 11' drivesthe screw nut 12' in the axial or Z-direction. The screw nut 12' ismounted on the plate 31' which is supported by a pair of guide shafts34' arranged in a housing 35' and linear bearings 36'. Plate 31'reciprocates the radial air bearing-supported spindle rotor assembly,via the rod 32' in the axial or Z-direction comprising the rotor shaft,the collet, and diaphragm, including the thrust bearing assembly,described in FIGS. 14 and 22.

The plate 31' also drives the pressure foot 20' supported and guided bya pair of brackets 37', air cylinders 16', swivel joints 17', and shafts19' located at both sides of the spindle unit 33 (of the type shown inFIGS. 14 and 22 as 14" and 14'", respectively) in a manner similar tothe conventional Z-axis units.

By virtue of the foregoing, the spindle saddle 13 and spindle body 4which constitute a significant weight or mass are not driven.Consequently, these mechanisms exceed an acceleration of 2G's with theresult that the feed/retract time for Z-axis movement can besubstantially reduced. In addition, the drilling of holes between about0.004" to 0.25" in diameter can now be achieved with this unit becausethe collet chucking force is maintained the same as in the conventionalrange of drill bit diameters.

While we have shown and described several embodiments in accordance withthe present invention, it should be clearly understood that the same issusceptible of changes and modifications without departing from thescope of the invention. Therefore, it is not intended that the inventionbe limited to the details shown and described herein but that it shouldcover all such changes and modifications falling within the scope of theappended claims.

We claim:
 1. A Z-axis unit, the Z-axis unit comprising a base, a drivemotor operatively associated with the base, a screw shaft adapted to bedriven by the motor, a screw nut operatively associated with the screwshaft, a plate to which the screw nut is connected, a push rodassociated with the plate, a spindle housing fixed with respect to thebase, a spindle rotor assembly, and air bearing means for mounting saidspindle rotor assembly for movement within and relative to the spindlehousing in response to actuation of the screw shaft, the screw nut, theplate and the push rod by the motor.
 2. The Z-axis unit according toclaim 1, wherein a pressure foot is operatively associated with anddriven by the plate in response to actuation of the motor, and means areprovided for actuating the pressure foot into position for clamping aworkpiece between the unit and a work table.
 3. The Z-axis unitaccording to claim 1, wherein the spindle rotor assembly comprises arotor shaft mounted in the housing and operatively arranged to move inan axial direction of the rotor shaft, means for moving the rotor shaftin the axial direction relative to the housing, a taper colletoperatively associated with the rotor shaft, and means for axiallymoving said taper collet relative to the shaft for enabling a selectiveengagement of a tool with sufficient chucking force for a matchingoperation and for enabling a disengagement of the tool during a toolchanging operation.
 4. The Z-axis unit the Z-axis unit comprises:a base;a drive motor operatively associated with the base; a screw shaftadapted to be driven by the motor; a screw nut operatively associatedwith the screw shaft; a plate to which the screw nut is connected; apush rod associated with the plate; a spindle housing fixed with respectto the base; a spindle rotor assembly including a rotor shaft mounted inthe housing and operatively arranged to move in an axial direction ofthe shaft, means for moving the rotor shaft in the axial directionrelative to the housing, a taper collet operatively associated with therotor shaft, and means for axially moving said taper collet relative tothe shaft for enabling a selective engagement of a tool with sufficientchucking force for a machining operation and for enabling adisengagement of the tool during a tool changing operation; air bearingmeans for mounting the rotor assembly for movement within and relativeto said spindle housing in response to actuation of the screw shaft, thescrew nut, the plate and the push rod by the motor; a pressure footoperatively associated with and driven by the plate in response toacutation of the motor; and means for actuating the pressure foot into aposition for clamping a workpiece between the unit and a worktable. 5.The Z-axis unit according to claim 4, wherein one end of the rotor shaftis provided with a thrust flange, and a thrust bearing assembly isrotatably arranged with respect to the housing and contains thrust airbearings for supporting the thrust flange.
 6. The Z-axis unit accordingto claim 5, wherein radial air bearings are located at axially spacedpositions along the rotor shaft.
 7. The Z-axis unit according to claim6, wherein the thrust bearing assembly has a cavity with a diaphragmarranged therein adapted to be pneumatically actuated, and a push rod isassociated with the diaphragm so as to cause the taper collet to moveaxially relative to the rotor shaft and to disengage the tool inresponse to pneumatic actuation of the diaphragm.
 8. The Z-axis unitaccording to claim 7, wherein the rotor shaft is a hollow shaft, thetaper collet has a portion slidable within the rotor shaft, and meansare arranged between the portion and an end of the shaft for biasing thetaper collet toward a position in which the collet engages the tool withsufficient chucking force to perform high quality machining operationsover a wide speed range.
 9. The Z-axis unit according to claim 8,wherein the thrust bearing assembly has a cavity with a diaphragmarranged therein adapted to be pneumatically actuated, and the push rodis associated with the diaphragm so as to cause the taper collet to moveaxially relative to the rotor shaft and to disengage the tool inresponse to pneumatic actuation of the diaphragm.
 10. The Z-axis unitaccording to claim 9, wherein the portion of the taper collet isarranged to be actuated by the push rod to disengage the tool bit duringthe tool changing operation.
 11. The Z-axis unit according to claim 10,wherein the rotor shaft has a built-in core approximately intermediatethe radial air bearings, and motor windings are arranged in the housingin operative arrangement with the core for driving the rotor shaft whenthe windings are selectively energized.
 12. The Z-axis unit according toclaim 11, wherein the diaphragm, thrust flange and rotor shaft are sizedin accordance with the air pressure supplied to the bearings and thediaphragm so as to permit reduction in the overall mass and improve therun-out characteristics of the spindle unit.